A temporal separation of helper T-cell activities involved in the induction of immunoglobulin secretion by B-cells has been demonstrated. There is an early T-cell requirement a day or two in advance if there is an appearance of secretory cells as well as a late requirement for T-cells in which B-cells are stimulated to enhanced secretion within minutes. The latter enhancement in the extracellular appearance of (3H)-immunoglobulins in response to thymocytes occurs under circumstances in which the amount of intracellular (3H)-immunoglobulin remains unchanged. Such thymus cell effects are specific in the sense that T-cells added to liver cells do not affect the rate of secretion of serum albumin. The secretory products of thymus cells in response to fetal calf serum and other mitogenic agents have been separated in part and materials secreted in specific response to concanavalin A and pokeweed mitogen observed.